A significant restraint in RF and microwave IC design stems from the difficulty in realizing an integrated passive inductor with sufficiently high Q over a broad bandwidth. Large space requirements, low inductance values and low Q factors make these inductors unsuitable for precision applications.
Active designs have allowed larger inductance values to be realized. However, the active inductors published to date are limited in that they are often not tunable. When inductance tuning is introduced, the Q factor usually shows a strong dependence on both the tuning parameter and the frequency of operation. As a result, tuning both the inductance and the Q factor requires an iterative tuning procedure.
A Q-enhancing technique has been described by Tokumitsu et al in [1]. In this design a cascode FET arrangement with resistive feedback is used such that when the FETs are matched, the active inductor's loss resistance can be canceled. The resistive feedback described in [1] was replaced with a common gate FET in [2] which offered improved Q factor. everHow, tuning of Q of the inductance was not easily accomplished.
Alinikula et al [3] described an alternative topology to that given in [2] which offered greater tuning flexibility. With this technique the effect of finite channel conductance, g.sub.ds, was examined and a design was proposed which minimized sensitivity to g.sub.ds. Using a FET operating in its linear region as a variable resistor, the frequency at which maximum Q occurred could be controlled. For narrow bandwidths the Q factor approached 500, however, the loss resistance showed a strong frequency dependence.
A resonator design described by Haigh [4] introduced tuning of both the resonant frequency and the Q factor. A resonant circuit was formed by using two integrators terminated in a capacitance and connected in a feedback loop. Although the resonant frequency remained independent of Q tuning, the circuit showed a large loss resistance for frequencies below the resonant frequency.
Tuning control of both inductance and Q factor was also reported in a topology proposed by Lucyszyn and Robertson [5]. This design simulated an inductance that was adjustable over a narrow range of values by changing the gate bias voltage of a single FET. The Q factor could also be tuned to be maximum at an arbitrary frequency. However, as with the previous design, the loss resistance showed an appreciable frequency dependence resulting in very narrow band performance.
A more recent design presented by Yong-Ho et al [6] expanded on a common Q enhancement technique using a single FET with lossy inductive feedback. Instead of using a passive feedback inductor, an active inductor circuit was used in this design. The inductance was made tunable over a wide range by varying the loss resistance of the active feedback circuit. Tuning of the Q factor was accomplished by varying the positive supply voltage for all FETs and could only be set to infinity for a narrow band of frequencies. The loss resistance also varied over a wide range for frequencies outside of this narrow band.
List of References
1. T. Tokumitsu, T. Tanaka, M. Aikawa, S. Hara, Broadband Monolithic Microwave Active Inductor And its Application to Miniaturized Wide-band Amplifiers, in IEEE Trans. Microwave Theory Tech., vol 36, pp. 1920-1924, December 1988. PA0 2. T. Tokurnitsu, M. Aikawa, S. Hara, Lossless, Broadband Monolithic Microwave Active Inductors, in IEEE MTT-S Symp. Dig., 1989, pp. 955-958 PA0 3. P. Alinikula, R. Kaunisto, K. Stadius, Q-Enhancing Technique for High Speed Active Inductors, in 1994 IEEE International Symposium on Circuits and Systems, pp. 735-738. PA0 4. D. G. Haigh, GaAs MESFET Active Resonant Circuit for Microwave Filter Applications, in IEEE Trans. Microwave Theory Tech., vol 42, pp. 1419-1422, July 1994. PA0 5. S. Lucyszyn, I. D. Robertson, Monolithic Narrow-Band Filter Using Ultrahigh-Q Tunable Active Inductors, in IEEE Trans. Microwave Theory Tech., vol 42, No. 12, pp. 2617-2622, December 1994. PA0 6. C. Yong-Ho, H. Song-Cheol, K. Young-Se, A Novel Active Inductor and Its Application to Inductance-Controlled Oscillator, in EEE Trans. Microwave Theory Tech., vol 45, No.8, pp.1208-1213, August 1997.